Other experts said the non-addicted siblings offered hope of new ways of teaching addicts "self-control".

It has long been established that the brains of drug addicts have some differences to other people, but explaining that finding has been more difficult.

Experts were unsure whether drugs changed the wiring of the brain or if drug addicts' brains were wired differently in the first place.

This study, funded by the Medical Research Council, attempted to answer that by comparing the brains of 50 cocaine or crack addicts with the brain of their brother or sister, who had always been clean.

Both the addicts and the non-addict siblings had the same abnormalities in the region of the brain which controls behaviour, the fronto-striatal systems.

The suggestion is that these brains may be "hard-wired" for addiction in the first place.

Lead researcher Dr Karen Ersche said: "It has long been known that not everyone who takes drugs becomes addicted."

Case Study

By David Shukman

Science Editor, BBC News

I met two of the participants in the study in Cambridge yesterday.

Sophia has the pallor and nervousness of a long-term user of cocaine and crack. Her elder sister Teresa is smartly-dressed and describes herself as a control freak.

They went through the tests and the scans and were surprised to find that they share the same abnormalities of the brain. It's a discovery that makes their contrasting lives all the more remarkable.

Sophia is receiving treatment but admits she has trouble with self-control. Theresa, with a similar biological predisposition to addiction, has found the strength of character to stay clean.

Poles apart, they are nevertheless devoted to each other and these findings bring them closer. A unique project has an unexpectedly moving outcome.

She told the BBC: "It shows that drug addiction is not a choice of lifestyle, it is a disorder of the brain and we need to recognise this."

However, the non-addicted siblings had a very different life despite sharing the same susceptibility.

"These brothers and sisters who don't have addiction problems, what they can tell us is how they overcome these problems, how they manage self-control in their daily life," Dr Karen Ersche said.

Dr Paul Keedwell, a consultant psychiatrist at Cardiff University, said: "Addiction, like most psychiatric disorders, is the product of nature and nurture.

"We need to follow up people over time to quantify the relative risk of nature versus nurture."

It is possible that the similarities in the sibling's brains may not be down to genetics, but rather growing up in the same household. Research on the relationship between addiction and the structure of the brain is far from over.

However, many specialists believe these findings open up new avenues for treatment.

"If we could get a handle on what makes unaffected relatives of addicts so resilient we might be able to prevent a lot of addiction from taking hold," said Dr Keedwell.

The chief pharmacist for Derbyshire Mental Health Trust, David Branford, said the study, "implies that addiction does not produce noticeable changes to brain structure and function which means that there may be provision for looking at new treatment techniques for addiction".

Prof Les Iversen, from the department of pharmacology at the University of Oxford, said: "These new findings reinforce the view that the propensity to addiction is dependent on inherited differences in brain circuitry, and offer the possibility of new ways of treating high-risk individuals to develop better 'self control'."